Ultrahigh-frequency mixing circuits



March 3, 1952 c. e. MONTGOMERY 2,539,843

ULTRAHIGH-FREQUENCY MIXING CIRCUITS Filed Jan. 3, 1946 2 SHEETS-SHEET l ATR II l2 XMTR LO MXR XFMR ROVR FIG. 4

I I ,50 52 I INVENTOR. CAROL G. MONTGOMERY ATTORNEY March 18, 1952 c. G. MONTGOMERY 2,589,343

ULTRAHIGH-FREQUENCY MIXING CIRCUITS INVENTOR. CAROL G.MONTGOMERY ATTORNEY Patented Mar. 18, 1952 ULTRAHIGH-FREQUENCY MIXING CIRCUITS Carol G. Montgomery, Belmont, Mass, assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application January 3, 1946, Serial No. 638,886

Claims. (01. 250-20) This invention relates generally to electrical apparatus involving ultra-high and microwave frequency energy and more particularly to frequency mixing circuits for use at these frequencies.

According to conventional usage, many radio receivers, such as used in broadcast, communication, or radar systems, employ the superheterodyne principle. In receivers which employ the superheterodyne principle, a local oscillator signal is combined in a mixing circuit with the incoming or received signal to produce a third or intermediate frequency signal. Many receivers, especially those used in communication and radar systems, have high sensitivity, and in these receivers noise becomes an especially important factor. One common source of noise is the local oscillator. This noise results from the combination in the mixing circuit of signals of two or more frequencies which are present in the-local oscillator signal. Another source of noise is that which is picked up by the antenna of the receiver. Signals having frequencies other than the frequency of the desired signal will combine in the mixing circuit with signals from the local oscillator and with other signals picked up by the antenna to produce undesirable noise.

Another characteristic of conventional mixing circuits used heretofore is that energy from the local oscillator is coupled to the circuit associated with the antenna, and as a result, the local oscillator signal is radiated by the antenna. In most applications of radio, either civilian or military, such coupling of energy from one source into another source is highly undesirable.

Apparatus known as a magic-T which is hereinafter described functions in a manner to overcome or obviate disadvantages heretofore encountered in signal mixers.

The magic-T is an electrical device having four terminal pairs in the instance of a network, or four branches in the instance of a wave guide structure. Hereinafter described in terms of a wave guide structure having four branches, the magic-T possesses many advantages which will become apparent from the following partial list of its characteristics:

(1) If the first and second branches are terminated in their characteristic impedances and energy is supplied to a third branch, energy is present in equal magnitude and in phase at the first and second branches. No energy appears at the fourth branch.

(2) If under-the conditions stated in (1) above,

energy is supplied to the fourth branch rather is supplied to the first and second branches,

energy is present at the third and fourth branches in respective magnitudes which are dependent upon the relative phase of the energy at the first and second branches.

(4) If energy is supplied to either the first or second branch, energy is present at the third and fourth branches. No energy appears at the remaining second or first branch.

Several structures embodying magic-Ts are disclosed in the copending applications of Robert H. Dicke: Serial No. 584,226, filed March 22, 1945, now Patent No. 2,547,378 granted April 3, 1951; and Serial No. 586,413, filed April 3, 1945.

It is an object of the present invention to provide apparatus for combining energy from two sources in such a manner that the resulting intermediate frequency energy output does not contain components which would ordinarily be produced by combination of energy of two frequencies originating in one of the energy sources.

Another object of this invention is to provide a mixing circuit wherein signal energy and local oscillator energy may be combined without causing radiation of energy which originated in the local oscillator.

It is a further object of the present invention to provide a structure equivalent in result, though different in form and principle of operation, to a magic-T; that is, which has the characteristics set forth above and which is simple in construction.

The mixing circuit of the present invention employs a section of wave guide in whichtwo modes of transmission are simultaneously present. A first coupling means is connected to the wave guide section in a manner to be electromagnetically coupled to said section for energy in one of said modes and to be essentially decoupled electromagnetically from said section for energy in the other of said modes. A second coupling means is connected to the wave guide section in a manner to be electromagnetically coupled to said section for energy in the other of said modes and to be essentially decoupled from said section for energy in said one mode. The first and second coupling means correspond to the third and fourth branches of the magic-T as described above.

asaaeas A pair of coupling means are oriented in the wave guide section in positions and in a way such that they are both electromagnetically coupled to said guide section for the energy in one of said modes in the same phase and for the energy in the other of said modes in 180 phase opposition. These latter coupling means correspond to the first and second branches of the magic-T as described above.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings in which:

Fig. 1 is a block diagram of a. radar or radio object-locating system in which the present invention may be utilized;

Fig. 2 is an isometric view of the preferred embodiment of the present invention;

Fig. 3 is an isometrieyiew of a simplified structure corresponding to the embodiment shown in Fig. 2; and i I i z 1 V Fig. 4 is a schematic illustration of the electric field distribution within thestructure of Fig. 3. Reference is now h d to he dye .55 answe particularly to Fig. ;1 thereof wherein is illustrated in block diagram form a radio objectlocating system whiclrthe present invention may be 'utilizedficomprising a transmitter- H which feeds anantenna er -other radiating device Iz through a suitable transmission-line I3.

Energy is coupled from the-transmission line I3 through a transmitreceive (TB) device 13 to a mixer I5. The-T'B deVice -M isessentially an amplitude discr minator which prevents trans- 7 mitted signals which are of greater amplitude than received signals, from reaching the mixer i5 in which the present invention is embodied.

An anti-transmit-receive (ATR) device I3 is connected to the transmission line I3 at a position between the transmitter I I and the TB. device IQ. The position of -the ATR device 13 is V such as to insure that a maximum amount of ceiver-23 containing the remainder of the system receiver which normally includes such components as amplifiers, a detector, and an indicator.

In the; operation; of the system of Fig. 1, a signal from thetransmitter I I is radiated-by the antenna ig. Signals received by the antenna iii pass through the TR device I4 to themixer I5.

wherein they are combined with a signal from the local oscillator M to produce signals-at the 1 intermediate frequency. These intermediate fre- I quency signals are amplifiectdetected and indicated in the conventional manner by the re-.

ceiver 23. I

Reference is now had to Fig. 2 of the drawings in which there is shown the preferred embodiment ofthe present invention, which is equivalent to amagic-T mixer and which comprises a section of rectangular wave guide 24 connected to a section of rectangular wave guide 26 which is in turn connected to a third section of rectangular wave guide 28. The mixer here shown is adapted to be used as the mixer I5 of a system as in Fig. i. In Fig. 2, signal sources and utilization circuits corresponding to those shown in Fig. l bear like reference numerals.-

The invention utilizes the}.principle thatl to.

transmit the dominant transverse electric mode or the TEo,1 mode, the major cross-sectional dimension of the rectangular wave guide need be only equal to or greater than one-half wavelength as measured at the operating frequency. To transmit the next higher transverse electric mode or the TEn,2 mode, the major cross-sectional dimension must be equal to or greater than a whole wavelength.

Referring again to Fig. 2, the wave guide 24 is designed to operate in the dominant or TEo,1 mode, and should therefore have a major crosssectional dimension which is between the limits of one-half and one wavelength as measured at the mid-operating frequency. The wave guide 28 is designed to operate in the next higher transverse electric mode or the TIT-0,2 mode, and its major cross-sectional dimension is therefore .between the limits of one wavelength and one and one-half wavelengths. 'Wave guide 23 may then also be excited in a TEo,1 mode. The section of wave guide 231s an odd number of quarter wavelengths long, and 'acts'as a coupling icr matching transformerbetween wave guidesifl. and 28. Wave guide23 is so designed that characteristic impedance isthe geometridrn'e'an of the characteristic impedances of the wave guides 24 and 23. A sectionof wave guide 33, also designed to operate in' the'TE0,1' "mode f and not to support the TEQaImOde, isjoined to the wave guide 28 in such a manner thatlals'ignal propagated in the wave guide 33 in the dominant mode will excite the waveguide 28 in the E One end of the wave guide 331s provided with a shorting plunger 32. The position cf the plunger 32 may be variable to permit adjustment of the coupling between the wave guide BQ'Iand the wave guide 23. The wave guide 33'iselectrically connected to the local oscillator 2 i. Two detectors 33 and 34 arerlocated within the wave guide 28 at positions between the wave guides 23 and 30. The detectors 33 and. 34' are so disposed in a plane perpendicular to the longitudinal'axis of the wave guide '28 that they areloc'ated'at the two maxima of the electric field existing"" in the wave guide 28 during its oper'ationin the TEo z. The detectors 33 and .34 may be'silicon crystals or other suitable devices possessing nonlinear voltage-current characteristics. Coaxial leads 35 and 36, associated respectively withfithe detectors 33 and 34 for applying a signal thereto and for extracting a signal therefrom, maylbe connected as shown to a transformer '22 which is in turn connected to the receiver 23.

Received signals are coupledintofthe wave guide 28 from the TR. device M in such a manner as to excite the TEo, mode therein. The signals from the TR. device I4 thus appearfat the detectors 33 and 34 in phase and in' equal magnitude. Because of the electrical fieldconsiderations involved. no signal intheTEoja'rnode is coupled into the waveguide 33; this follows from the fact that guide 30 is coupled .to guide 23 over a distance along guide 30. of an odd.number of half wavelengths and mostener gylffigm pled from the TE0,1 mode in guide 213 'to" guide 30 will cancel itself out. Hencejsubsta'ntially none of the received signal'is lost 'inthejloc'al oscillator. The signal from the'localdscillator 2i is propagated through'wave guide '3 3Tand coupled into the wave guide i23fin' such almanner as to excite the wave guide 28f intheTEca mode. The'elect'ric field configiir ationfofithis mode is such that signalsorigin'thg.

and in equal magnitude. The TEo,2 mode signal in wave guide 28 does not excite the TEo,1 mode in wave guide 24 and since guide 24 will not propagate energy in the TEo,2 mode, substantially none of the local oscillator signals can be communicated to the TR. device 14 and the antenna to cause radiation of the local oscillator signal.

Under the conditions here set forth, namely, that detectors 33 and 34 receive signals in phase from one source and signals in 180 degree phase opposition from a second source, intermediate frequency signals appear on leads 35 and 36 and are in 180 degreephase opposition if the detectors 33 and 34 are symmetrically connected to coaxial leads 35 and 33, respectively. If the detectors 33 and 34 are asymmetrically connected to their respective coaxial leads, the intermediate frequency signals will be in phase and they may be directly combined. In the embodiment here described, the intermediate frequency signals from the detectors 33 and 34 are out of phase and are combined in the transformer 22. The transformer 22 may be a half-wavelength section of transmission line; one intermediate frequency signal may be applied to such a section of transmission line, and the resulting phase-reversed signal is then directly combined with the other intermediate frequency signal.

Many other devices for combining signals are known in the art. Transformer 22 may, for example, be of the conventional air-core type.

When signals from a single source combine at the detectors to produce undesired noise, the phase of the intermediate frequency output noise will be such that, regardless of the type of transformer used, the noise signals will be cancelled out.

In Fig. 2, the positions of the local oscillator 2| and the TR device l4 may be interchanged if desired. That is, the TR device 14 may be coupled into wave guide 38 instead of into wave guide 24, and the local oscillator 2i may be coupled into wave guide 24 instead of into Wave Another modification is that in which guide 38. local oscillator 2| and TR device G4 are connected to couple energy into wave guide 28 at the points at which the detectors 33 and 34 are shown in Fig. 2, and the detectors 33 and 34 are then appropriately placed in the wave guides 24 and 38, respectively. Therefore, although Fig. 2 illustrates the preferred embodiment of the present invention, the invention is not to be construed to be limited to the particular arrangement shown and described herein in detail.

Reference is now had to Fig. 3 which is an isometric view of a simplified structure corresponding to that shown in Fig. 2, comprising a section of wave guide 31 to which there are connected coupling probes 33, 48, and 42. The probes 38, 48 and 42 may be replaced by coupling loops or any other coupling means well known in the art. The wave guide 31 is terminated at both ends in its characteristic impedance, and is designed to operate in both the TEo,1 and 'I'Eo,z modes.

The probe 38 is located near one end of the wave guide 31 for excitation of the wave guide in the TEo,1 mode. The coupling probes 48 and 42 are disposed in a plane perpendicular to the longitudinal axis of the wave guide 31, and at maxima of the electric field set up in the wave guide 31 during its excitation in the TEo,2 mode. A composite coupling 44 is connected to the wave guide 31 and comprises two coupling probes 46 and 48 which are located at electric field maxima of the TEm mode. The probes 46 and 48 are electrically connected together by a section of coaxial transmission line, the electrical length of which is an odd number of half wavelengths.

Probes 48 and 42 correspond to the probes 35 and 38 of Fig. 2. Probe 38 corresponds to the wave guide 24, and the composite coupling 44 corresponds to the wave guide 38 shownin the magic-T structure of Fig. 2. The relative placement of the probes 38, 48, 42, and composite coupling 44 along the longitudinal axis of the wave guide 31 is immaterial. Therefore, although the probes 48 and 42 are shown between the probe 38 and composite coupling 44 in the simplified magic-T of Fig. 3, the invention is not to be so limited.

.A signal applied to the coupling probe 38 excites the wave guide 31 in the TEo,1 mode. The TEo,1 mode electric field distribution within the wave guide 31, as seen in a plane or planes parallel to that containing probes 48 and 42, is substantially as shown by the curve 58 of Fig. i.

In Fig. 4 are also shown the relative lateral positions of probes 38, 48 and 42. Probes 46 and 48 lie directly behind probes 48 and 42, respectively, and are therefore here hidden from view. Referring now to Figs. 3 and 4, the field strength and signals at the probes 48 and 42 due to a signal applied at probe 38 are seen to be equal and in phase. The signal set up in the wave guide by the probe 38 also induces signals in the probes 46 and 48 which are equal in magnitude and in phase, but because the coaxial line connecting probes 48 and 48 has a length equal to an odd number of electrical half wavelengths, the resultant signal output from the composite coupling 44 is zero. This condition will be recognized as conforming to characteristic (1) of a magic-T as listed above.

A signal which is fed to the composite coupling 44 causes out of phase voltages at probes 46 and 48 and excitation of the wave guide 31 in the TEo,2 mode, because of the odd number of electrical half wavelengths by which the probes 48 and 48 are separated. This can more readily be seen by reference to Fig. 4 in which the curve 52 represents the electric field distribution for the TEo,2 mode. The corresponding signals at the probes 48 and 42 will be equal in magnitude and out of phase. The signal picked up by the probe 38 will be zero because the electric field distribution for the TE'0,2 mode passes through zero at this point. The structure of Fig. 3 thus further operates in accordance with characteristic (2) of a magic-T as listed above.

A signal applied to either the probe 48 or ,42 excites the wave guide 31 in both the TEo,1 and TE modes. The electric field distributions or the TEo,1 and TEo,2 modes are represented by the curves 58 and 52, respectively, in Fig. 4, for the instance in which a signal is applied to the probe 48. From Fig. 4 it is seen that the magnitude of the electric field at the remaining probe 42 due to the TEo,1 mode is equal but opposite in polarity to the magnitude of the electric field due to the TEo,2 mode, and thus no signal appears at the remaining probe 42. Since both the TEo,1 and TE0,2 modes are excited in the wave guide 31, the signal is coupled into both the probe 38 and the composite coupling 44. This corresponds to characteristic (4) of a magic-T as listed above.

Signals to both probes 48 and 42 excite the wave guide 31 in one or both of the modes T'Eo,1

7 and TEo,z. The relative amplitudesof the two "modes excited "will be dependent upon the relative phases of the signals applied to the probes 40 and ""42, and, therefore, the signals which appear at probe 38 and composite coupling i are dependent upon the relative phase of the signals at -the probes diland-fl. This latter characteristic thus correspondsto' characteristic (3) -of'a magic-T as listed above.

The simplified structure 'shown 'in- Fig; 3, corresponding to the structure ofFig. 2,-and the acc'ompanying explanatory diagram of Fig. 4 have here been described for the purpose of providing a simplified explanation of the manner in *which -the mixer embodying my invention functions.

While therehas been described'what is at present considered-to be the preferred embodiment of the invention, it will be obvious-to those skilled "in the art thatvarious changes and modifications may be made thereinwithout departing from the invention.

'What is claimed is:

1. In a receiver of the-superh'eterodyne type, a --mixing circuit for combining a receive-:1 radio frequency signal with a local oscillator radio frequency signal -'to provide an intermediate frequency signal; said mixing circuit comprising a rectangular wave guide; means'positioned in said "wave guide for exciting said wave guide in the TED; mode with the received signal, means positioned in said wave guide for exciting said wave "guide in the TEcs mode with the local'oscillator "-signaL first and-second detector means so positioned'ins aid wave guide as to be excited in phase 1 by said received signal and in ISO-degree phase opposition by said local oscillator -signal and means for combining the output of-=said-first and 1 second detector means to provide an intermediate frequency signal.

2.'A mixing circuit comprising a section of rectangular wave guide adapted to propagate sig- *nals in theTEai and TEO,2 modes, first-means "forcouplingasignal into saidwave guide in such a manner as to excite the TE -mode therein,

" second means for "couplinga signal into said wave guide in-such a manneras to excite the TEmmode therein; and third and fourth means forcoupling-signals from said wave guide which are excited-by both the TEu,1 andTEazmodes,

i'said third and fourth coupling means beingdis- Ti posed in a plane perpendicular to the longitudinal "axis of said wave guide and being positioned in said plane at respective maxima of the electrical -field configuration when said wave guideis operating in the TE mode.

3. A- mixing circuit for obtaining-a beat frequency signalfrcm two high frequency signals comprising arectangular wave guide designed to propagate the highifrequency signals in the TEo,1

- and the TEo,2 mod'esjmeans coupled tosaid guide for exciting said guide with one of said high fre- 1 quency signals in the TEO,1 mode, means'coupled -to said guide-for exciting said guidewith the otherof said high frequency signals in the 'IEo,2 rno'de,-a pair of detector means each coupled to said guide ina'manner to be excited bysaid one "-signal inphase "and by said other signal in 180 phase "opposition, and means coupling the output fcircuits of said detector means together in a ii l Obtain a ibeatfrequency signal only from the combining of thezsignalsaonerfromnafih sectional dimension greaterthan \/2,:and less than A and being coupled tosubstantiallythe center of one end ofsaidfirstwave guide; a'third wave guide for applying the otheriofsaid-ihigh frequency signals to said first 'wave .tguidasaid third waveguide'having a major cross-sectional dimension greater than M2; and less than wand being coupled to the otherend of:said first'wave guide at two points spaced substantially: an odd number of quarterwavelengths-apartalong the major cross-sectional aiXiSlOf saidthird wave guide and symmetrically "with respect to the minor cross-se'ctionalaxis thereof, a :pairof detectors coupled to said first wave guide in acrosssectional plane of said first-Waveguide and at points spaced symmetrically: withrespect to the minor cross-sectional axis-10f said first wave guide, and means coupling the outputs of said detectors together in a manner to obtain a beat frequency signal only from the combining of said two high frequenoysignals.

EQA mixing circuit forobtaining-a beat-frequency from two high frequency circuits comprising a rectangular wave guidadesignedwto propagate said high frequency signalsin at least two different modes of transmissiongafirstmeans coupled to said guide in a manner to be electromagnetically coupled thereto for the energy in one of said modes and to be substantially-electromagnetically decoupled therefrom for energy in the other of said modes, asecond means coupled to said guide in a manner to rhe -electromagnetically coupled thereto fort-heenergy in-,=:the other of said modes and to-be substantially-electromagnetically decoupled therefrom in-s'aid one of said modes, third and fourth means; coupled to said guide in a manner to beelectromagnetically coupled thereto for the energy in nsaid one of said modes in the-same phase andiforthe energy in said other'of said modes in 180 phase opposition, fifth means for applying a first high frequency signal in said-one mode=to-said:fir.st means, sixth means for-applying a second high frequency signal insaid other mode tosaidsecond means, a first :de-tectingmeanscoupled to said third means, asecond detecting means cou- V pled to said fourth means andoutput meanscou- EEFERENCESQCITED The following references are ofsrecord in-the file of this patent:

UNITED STATES PATENTS Name 7 .1 Date Ginzton Oct. ,1, 1946 Number 

